Geochemical evidence for strong ongoing methanogenesis in Sanhu region of Qaidam Basin

Sanhu depression of Qaidam Basin is the largest biogenic gas production region in China.Headspace samples were collected from two wells in this region,and hydrogen and propylene compounds were detected in these samples with a certain concentration.The stable hydrogen isotope ratio of H 2 is relatively light (-700‰--820‰).The stable carbon isotope ratio of propylene ranges from -27‰ to -40‰,which coincides with the rule of change of the stable carbon isotope of kerogen at the corresponding horizon.The charac...     

Diagenetic fluid evolution and water-rock interaction model of carbonate cements in sandstone: An example from the reservoir sandstone of the Fourth Member of the Xujiahe Formation of the Xiaoquan-Fenggu area,Sichuan Province,China

Carbonate cement is the most abundant cement type in the Fourth Member of the Xujiahe Formation in the Xiaoquan-Fenggu area of the West Sichuan Depression. Here we use a systematic analysis of carbonate cement petrology, mineralogy, carbon and oxygen isotope ratios and enclosure homogenization temperatures to study the precipitation mechanism, pore fluid evolution, and distribution of different types of carbonate cement in reservoir sand in the study area. Crystalline calcite has relatively heavy carbon and oxygen isotope ratios(δ13C = 2.14‰, δ18O = -5.77‰), and was precipitated early. It was precipitated directly from supersaturated alkaline fluid under normal temperature and pressure conditions. At the time of precipitation, the fluid oxygen isotope ratio was very light, mainly showing the characteristics of a mixed meteoric water-seawater fluid(δ18O = -3‰), which shows that the fluid during precipitation was influenced by both meteoric water and seawater. The calcite cement that fills in the secondary pores has relatively lighter carbon and oxygen isotope ratios(δ13C = -2.36‰, δ18O = -15.68‰). This cement was precipitated late, mainly during the Middle and Late Jurassic. An important material source for this carbonate cement was the feldspar corrosion process that involved organic matter. The Ca2+, Fe3+ and Mg2+ ions released by the clay mineral transformation process were also important source materials. Because of water-rock interactions during the burial process, the oxygen isotope ratio of the fluid significantly increased during precipitation, by about 3‰. The dolomite cements in calcarenaceous sandstone that was precipitated during the Middle Jurassic have heavier carbon and oxygen isotope ratios, which are similar to those of carbonate debris in the sandstone(δ13C = 1.93‰, δ18O = -6.11‰), demonstrating that the two are from the same source that had a heavier oxygen isotope ratio(δ18O of about 2.2‰). The differences in fluid oxygen isotope ratios during cement precipitation reflect the influences of different water-rock interaction systems or different water-rock interaction strengths. This is the main reason why the sandstone containing many rigid particles(lithic quartz sandstone) has a relatively negative carbon isotope ratio and why the precipitation fluid in calcarenaceous sandstone has a relatively heavier oxygen isotope ratio.     

Evolution of strontium isotopic composition of sea-water from Late Permian to Early Triassic based on study of marine carbonates,Zhongliang Mountain,Chongqing,China

Collected from a Late Permian to Early Triassic sedimentary section in the Zhongliang Mountain of Chongqing, Southwest China, sixty marine carbonate samples were measured for the 87Sr/86Sr ratios, and corresponding evolution curve was constructed. The concentrations of SiO2, CaO, MgO, Mn and Sr are used to evaluate reservation of strontium isotopic composition for original seawater and the credi-bility of the dissolution method for sample preparation. The results show that most of the samples (except seven samples with the Mn/Sr ratios higher than 2) contain the original geochemistry signa-tures of ancient seawater. Compared to the published 87Sr/86Sr ratios from the Late Permian to Early Triassic, our database reported here is the largest and the curve constructed is the most complete. The strontium isotopic curve from Late Permian to Early Triassic is consistent globally and exhibits a gen-eral trend of steady increase during this period. The minimum of 87Sr/86Sr ratios (0.707011) occurs in the Late Permian (30 m in thickness below the Permian-Triassic boundary), and the maximum (0.708281), near the Early-Middle Triassic boundary. The lack of land plants and the rapid continental weathering result in the increase of 87Sr/86Sr ratios during the interval. The Permian-Triassic boundary in Zhongli-ang Mountain Section has been accepted internationally. The 87Sr/86Sr ratios of six samples near the boundary vary from 0.70714 to 0.70715 with an average of 0.70714, which is consistent with the value of 0.70715 (samples are from articulate brachiopod shells) from Korte et al. published in 2006 (within the error range in experiment). Accordingly, the strontium isotope composition in the Permian-Triassic boundary in this paper is of global significance. It can be confirmed that the 87Sr/86Sr ratios of the sea-water in the Permian-Triassic transition are in the range of 0.70714―0.70715.     

Discovery of SPICE and carbon isotope stratigraphic correlation of the Cambrian Furongian Series in Tarim Craton,NW China

The Furongian Series of the Cambrian in the Tarim Craton(NW China) is mainly composed of crystalline dolostones,and is an important target for oil and gas exploration. The chronostratigraphic framework of the Furongian Series in the Tarim Craton has not yet been established due to the scarcity of fossils and the absence of the Steptoean Positive Carbon Isotope Excursion(SPICE), which is the primary global carbon isotope excursion of the Furongian Series. In this study, the SPICE was discovered by examining the carbon isotopes of the representative drilling wells in the West Platform and East Basin. A positive carbon isotope anomaly with δ~(13)C values ranging from 1.5‰ to 1.9‰(PDB), were found in the middle of the Xiaqiulitag Formation of the three drilling wells in the West Platform, and a positive excursion with δ~(13)C values up to 4.0‰(PDB), which is correlated with the SPICE, was found in the lower limestone of the Tuershaketag Formation in the East Basin. Based on the carbon isotopic data and chemostratigraphic correlation, the chronostratigraphic framework of the Furongian Series across different facies in the Tarim Craton is preliminarily established. The basal boundary of the Furongian Series was preliminarily defined by the occurrence of the SPICE. Aweak positive anomaly of δ~(13)C was observed at the base of the Penglaiba Formation of the three boreholes in the West Platform and the top of the Tuershaketag Formation of Tadong-2 well. We propose that the positive anomaly of the carbon isotope above the weak negative anomaly at the base of the Penglaiba Formation and the top of the Tuershaketag Formation can be used to define the Cambrian-Ordovician boundary in the Tarim Craton.     

Origin of the Dashuigou independent tellurium deposit at Qinghai–Xizang Plateau: constraints from the light stable isotopes C,O, and H

By studying the light isotopic compositions of carbon, oxygen, and hydrogen, combined with previous research results on the ore-forming source of the deposit, the authors try to uncover its metallogenic origin. The δ¹⁸O and δ¹³C isotope signatures of dolomite samples vary between 10.2 and 13.0‰, and between-7.2 and-5.2‰, respectively, implying that the carbon derives from the upper mantle. δD and δ¹⁸O of quartz, biotite, and muscovite from different ore veins of ...     

Stable isotope paleohydrology and chemostratigraphy of the Albian Wayan Formation from the wedge-top depozone,North American Western Interior Basin

Understanding of the role of atmospheric moisture and heat transport in the climate system of the Cretaceous greenhouse world represents a major challenge in Earth system science.Stable isotopic paleohydrologic data from mid-Cretaceous paleosols in North America,from paleoequatorial to paleoArctic latitudes,have been used to constrain the oxygen isotope mass balance of the Albian hydrologic cycle.Over the range from 40°-50°N paleolatitude,sideritic paleosols predominate,indicating paleoenvironments with positive precipitation-evaporation(P-E)balances.Local exceptions occur on leeward side of the Sevier Orogen,where calcic paleosols in the wedge-top depozone record paleoenvironments with negative P-E balances in the orographic rain shadow.Stratigraphic sections in the Wayan Formation of Idaho(WF)were sampled from the wedge-top depozone.The units consist of stacked m-scale mudstone paleosols separated by m-scale sandstone-siltstone beds.Sections were sampled for organic carbon isotope profiles,and B-horizons from 6 well-developed paleosols were sampled for detrital zircons to determine maximum depositional ages.The first of these from the WF has produced a U-Pb concordia age of 101.0±1.1 Ma.This same WF section has produced a stratigraphic trend of upwardly decreasingδ~(13)C values ranging from-24‰upwards to-27‰VPDB,suggesting correlation to the late Albian C15 C-isotope segment.Pedogenic carbonates from the WF principally consist of micritic calcite,with carbon-oxygen isotope values that array along meteoric calcite lines(MCLs)withδ~(18)O values that range from-9.47‰up to-8.39‰VPDB.At approximately 42°N paleolatitude,these MCL values produce calculated paleoprecipitation values of-8.12‰to-7.04‰VSMOW,a range that is consistent with the estimates produced from other proxies at the same paleolatitudes across North America.These results indicate that despite the orographic rain shadow effect,the processes of meridional atmospheric moisture transport in this locale were similar to those in more humid mid-latitude paleoenvironments elsewhere in the continent.     

Origin of CO_2 in natural gas from the Triassic Feixianguan Formation of Northeast Sichuan Basin

The natural gas from the Triassic Feixianguan Formation of Northeast Sichuan Basin contains high H2S whereas relatively low CO2 concentrations and the CO2 display high δ13C values (ranging from -5.81‰ to 3.3‰ (PDB)). This seems to contradict the conventional wisdom that TSR should be a primary source of CO2 in natural gas from the Feixianguan Formation. In contrast, many authigenic calcite samples from these sites display very low δ13C values (ranging from -18.4‰ to -10.3‰ (PDB)). This suggests that the carbon from TSR source dominated the formation of calcite whereas the carbon from inorganic source came into CO2 in natural gas. In order to assess the origin of CO2 from these H2S-rich sites, we have calculated the relative contributions of organic and inorganic carbon sources to the CO2 and authigenic calcite. The organic carbon source possibly originated from TSR, whereas the inorganic one might be generated from marine carbonates dissolution. This calculation is based on the carbon isotopic compositions of CO2 and authigenic calcite as well as an isotopic mass balance. The results show that the contribution of organic carbon source to the CO2 is only 2%, whereas that to authigenic calcite is as high as 43% on average. Such results combined with thermodynamic evidence indicate that the isotopically light CO2 produced by TSR process may contribute to authigenic calcite precipitation during burial diagenesis. Distinguishable from Ordovician reservoir of Tarim Basin, Feixianguan reservoir of Northeast Sichuan Basin experienced rapid tectonic uplift due to Yanshanian movement after TSR occurred. Such tectonic event could induce temperature decrease and further promote carbonates dissolution. During these processes, secondary porosity has developed in Feixianguan carbonate reservoirs. Therefore, much attention should be paid to the structural highs in search of high quality carbonate reservoirs.     

Fingerprinting hydrothermal fluids in porphyry Cu deposits using K and Mg isotopes

In this study, we performed an integrated investigation of K and Mg isotopes in hydrothermally altered rocks from the giant Dexing porphyry Cu deposit in China. Both the altered porphyry intrusion and the surrounding wall rocks exhibit large variations in K and Mg isotope compositions, with δ41K values ranging between-1.02‰ and 0.38‰, and δ26Mg values ranging between-0.49‰ and 0.32‰. The δ41K and δ26Mg values of the majority of altered samples are higher than the isotopic baseline values for upper continental crust. We attribute the general increase in δ41 K and δ26Mg in altered rocks to hydrothermal alteration,which caused preferential incorporation of heavy K and Mg isotopes in alteration products, particularly phyllosilicates. However,a few altered samples show anomalously low δ41K and δ26Mg values. The δ41K and δ26Mg values do not correlate with K and Mg concentrations, or mineralogy of altered samples. The variable K-Mg isotope data likely reflect fluids of different physicalchemical properties, or different isotopic compositions. Based on the combined K-Mg isotope data, at least three groups of hydrothermal fluids are distinguished from the Dexing porphyry deposit. Therefore, K-Mg isotopes are potentially a novel tracer for fingerprinting fluids in complex hydrothermal systems.     

Detection and indication of 1,3,4-C27–29 triol in the sediment of northern South China Sea

After the detection of the 1,20,21-C_(29) long-chain triol in some sediments and freshwater pteridophytes, in this study, a new homologous long-chain triol, 1,3,4-C_(27-29), is detected for the first time in the Site4 B core sediment in the northern South China Sea. The hydroxyl location and length of the carbon chain of this newly discovered triol differ from those of 1,20,21-C_(29) triol. The test results of its molecular distribution and individual carbon isotope reveal that 1,3,4-C_(29) triol has a good correlation with n-C_(26-30) even carbon-numbered long-chain fatty alcohols, with R~2(n=68) values of 0.905, 0.929 and 0.903, respectively, and its carbon isotope composition, at –32.3‰±1.9‰, is similar to that of n-C_(26-30), at –29.13‰±0.87‰, –32.98‰±1.28‰, and –32.98‰±1.28‰. 1,3,4-C_(29) triol from the Site4 B core sediment and terrigenous long-chain fatty alcohol(n-C_(26-34)) show highly consistent distribution trends in the entire section; thus, the former could serve as a proxy indicator of the terrigenous input. Considering that the 1,20,21-C_(29) triol in previous research belongs to Azolla, which are fresh water pteridophytes, the 1,3,4-C_(27-29) triol identified in this study might have similar biogenetic derivation. Thus, determination of its biogenic area and growing environment could provide potential organic geochemical evidence supporting the terrigenous input and source in the northern South China Sea.     

Evaluation of the carbon isotopic effects of NDIR and CRDS analyzers on atmospheric CO2 measurements

Non-dispersive infrared(NDIR) and cavity ring-down spectroscopy(CRDS) CO_2 analyzers use 12CO_2 isotopologue absorption lines and are insensitive to all or part of other CO_2-related isotopologues. This may produce biases in CO_2 mole fraction measurements of a sample if its carbon isotopic composition deviates from that of the standard gases being used. To evaluate and compare the effects of carbon isotopic composition on NDIR and CRDS CO_2 analyzers, we prepared three test sample air cylinders with varying carbon isotopic abundances and calibrated them against five standard cylinders with ambient carbon isotopic composition using CRDS and NDIR systems. We found that the CO_2 mole fractions of the sample cylinders measured by G1301(CRDS) were in good agreement with those measured by Lo Flo(NDIR). The CO_2 values measured by both instruments were higher than that of a CO_2 isotope measured by G2201i(CRDS) analyzer for a test cylinder with depleted carbon isotopic composition δ~(13)C =-36.828‰, whereas no obvious difference was found for other two test cylinders with δ~(13)C=-8.630‰ and δ~(13)C=-15.380‰, respectively. According to the theoretical and experimental results, we concluded that the total CO_2 mole fractions of samples with depleted isotopic compositions can be corrected on the basis of their 12CO_2 values calibrated by standard gases using Lo Flo and G1301 if the δ~(13)C and δ18O values are known.     

Paleotemperature variations at Lake Caohai,southwestern China,during the past 500 years:Evidence from combined δ~(18)O analysis of cellulose and carbonates

The oxygen isotopic values of aquatic plant cellulose and carbonates in Lake Caohai sediments were measured using a continuous flow isotopic ratio mass spectrometer(CF-IRMS).Because of predictable oxygen isotopic fractionation between cellulose and its source water,the oxygen isotopic composition of paleo-lake water has been established quantitatively.Combined oxygen isotopic values of cellulose and carbonates were used in the‘Craig’equation to determine paleotemperatures and their variation in the lake during the past 500 years.Results show that the paleotemperature trend correlates well with meteorological records from Weining.There are four notable cold intervals at Lake Caohai over the past 500 years,namely 1540–1570AD,1670–1715AD,1780–1870AD and 1900–1930AD,and the former three cold intervals have been observed in the conventional Little Ice Age(LIA).These cold periods at Lake Caohai correspond well with those recorded from tree ring,peat,and ice core data from adjacent regions,particularly temperature those inferred fromδ18O of peat cellulose from Hongyuan Southwestern China.The trend in paleotemperature variations at Lake Caohai are also consistent with both the change of Indian summer monsoon,derived fromδ18O values of a stalagmite in Dongge,and a recorded shift in solar activity.The findings of this study illustrate that coupled analysis ofδ18O values of cellulose and carbonates from lake sediments may be used as a paleotemperature proxy.These results also provide further evidence of the existence of LIA in southwestern China.     

Variations of methane stable isotopic values from an Alpine peatland on the eastern Qinghai-Tibetan Plateau

Isotopic signature is a powerful tool to discriminate methane(CH_4) source types and constrain regional and global scale CH_4 budgets.Peatlands on the Qinghai-Tibetan Plateau are poorly understood about the isotopic signature of CH_4 due to the limited experimental conditions.In this study,three campaigns of diurnal air samples spacing 2-3 h were taken from an alpine peatland on the eastern Qinghai-Tibetan Plateau to investigate its source signal characteristics.Both CH_4 concentration and its stable carbon isotope(δ~(13)C-CH_4) were measured to derive the carbon isotopic signature of the CH_4 source using the Keeling plot technique.Diurnal variation patterns in CH_4 concentration and δ~(13)C-CH_4 were observed during summertime,with depleted δ~(13)C-CH_4 signals and high CH_4 concentration appearing at nighttime.The δ~(13)C-CH4 signature during summer was calculated to be-71 % ± 1.3%,which falls within the range of other wetland studies and close to high-latitude peatlands.The boundary layer dynamic and CH_4 source were supposed to influence the measured CH_4 concentration and δ~(13)C-CH_(4.)Further investigations of CH_4 isotopic signals into the nongrowing season are still needed to constrain the δ~(13)C-CH_4 signature and its environmental controls in this region.     

Evolution and global correlation for strontium isotopic composition of marine Triassic from Huaying Mountains,eastern Sichuan,China

Strontium isotope stratigraphy (SIS) has progressively become an efficient chemostratigraphic tool in the research and correlation of global geological events, such as global sea level fluctuations, orogeny, and paleoclimatic and paleoenvironmental changes. In this paper, 87Sr/86Sr ratios of the Triassic marine carbonate rock samples from Huaying Mountains, eastern Sichuan are measured, and the corre-sponding Sr-isotopic curve is constructed, based on the fundamental principle of strontium isotope stratigraphy, analysis of rock fabric, luminescent intensity, chemical composition and representative evaluation for the coeval seawater information. The 87Sr/86Sr ratios show a rapid rise from 0.70721 near the Permian-Triassic transition to 0.70830 at the end of Early Triassic, and then they decline rapidly to 0.70787 in the early Middle Triassic. These data are generally coincident with 87Sr/86Sr ratios of coeval seawater from previous papers, and the curve is also similar to other previous curves. This indicates that the global geological events are the most important controlling factors to the strontium isotope evolution of the global seawater in the Early and Middle Triassic.     

Natural gas geochemistry and its origins in Kuqa depression

According to gas compositional and carbon isotopic measurement of 114 gas samples from the Kuqa depression,accumulation of the natural gases in the depression is dominated by hydrocarbon gases, with high gas dryness(C1/C1-4)at the middle and northern parts of the depression and low one towards east and west sides and southern part.The carbon isotopes of methane and its homologues are relatively enriched in 13 C,and the distributive range ofδ13C 1 ,δ13C 2 andδ13C 3 is-32‰―-36‰,-22‰―-24‰and-20‰―-22‰,respectively.In general,the carbon isotopes of gaseous alkanes become less negative with the increase of carbon numbers.Theδ13C CO2 value is less than-10‰in the Kuqa depression,indicating its organogenic origin.The distributive range of 3 He/ 4 He ratio is within n×10-8 and a decrease in 3 He/ 4 He ratio from north to south in the depression is observed.Based on the geochemical parameters of natural gas above,natural gas in the Kuqa depression is of characteristics of coal-type gas origin.The possible reasons for the partial reversal of stable carbon isotopes of gaseous alkanes involve the mixing of gases from one common source rock with different thermal maturity or from two separated source rock intervals of similar kerogen type,multistages accumulation of natural gas under high-temperature and over-pressure conditions,and sufficiency and diffusion of natural gas.     

Carbon and nitrogen isotopes analysis and sources of organic matter in the upper reaches of the Chaobai River near Beijing,China

The carbon and nitrogen isotopes in the surface sediments,plants,and soil in the upper reaches of the Chaobai River have been researched.The results showed 27.75‰-21.58‰ and 1.32‰-6.74‰ for carbon and nitrogen isotopic ratios in the surface sediments,respectively.The sources of sedimentary organic matter in this area are soil organic matter,aquatic vascular plants,and riverine plankton,respectively,and a significant contributor to sediment in the Chaohe River,the Baihe River,and the Miyun Reservoir areas is soil organic matter.Furthermore,part of sedimentary organic matter in the Miyun Reservoir is attributed to the input from the Chaohe River and the Baihe River,the other is from C4 vegetation growing around individual point stations at the Miyun Reservoir.Compared with the situation in Hebei Province,the contribution of soil organic matter decreased significantly and river plankton and aquatic vascular plants increased significantly in Beijing municipal areas.This study reveals that the source of organic matter has a close relationship with the soil erosion.     

Constraints on the Ediacaran-Cambrian boundary in deep-water realm in South China: Evidence from zircon CA-ID-TIMS U-Pb ages from the topmost Liuchapo Formation

The placement of the Ediacaran-Cambrian boundary in deep-water realm in South China and the high-precision temporal framework for the Ediacaran-Cambrian transition in this region have not yet been completely solved. Recently, we have found two K-bentonite beds in the top of the Liuchapo Formation at the Pingyin section, Guizhou Province. It provides an opportunity for constructing the temporal framework of the transitional strata on the Yangtze Platform in South China and for determining the Ediacaran-Cambrian boundary in this area. In this study, we conducted high-precision CA-ID-TIMS U-Pb dating on zircons from the two K-bentonites. The ages of the two K-bentonites were precisely constrained at 536.40±0.47/1.1/1.2 Ma(2σ, n=7, MSWD=0.92) and 541.48±0.46/1.1/1.2 Ma(2σ, n=8, MSWD=1.3). Combining the geochronological results with organic carbon isotope data of chert in the topmost Liuchapo Formation from the section, we suggest that the EdiacaranCambrian boundary should be consistent with a significant negative organic carbon isotope excursion between the two Kbentonites. The scheme of the Ediacaran-Cambrian boundary in this study is of great significance for global correlation, and further for unravelling the information of the terminal Ediacaran-early Cambrian ocean.     

Hydrogen isotopes in palmitic and stearic acids in suspended particles from the Changjiang River Estuary

Hydrogen isotopes in lipid biomarkers can trace past changes in the hydrologic cycle. Recent studies have revealed the potential of hydrogen isotopes in microalgal lipids for quantitatively reconstructing water δ~2H(δD) values and salinity. In this study we collected suspended particles along a salinity gradient from the Changjiang River Estuary(CRE), and measured δD values in fatty acids in these particles. The results indicated that δD values of water were correlated highly with salinity from the CRE, in agreement with the results from other estuaries. δD values in palmitic acid and stearic acid had a positive correlation with δD values of water from the CRE. Nevertheless, in the CRE, hydrogen isotope fractionation in fatty acids relative to water increased as salinity increased, opposite the trend in hydrogen isotope fractionation with salinity found in microalgal culture and field studies. We attribute the increase in hydrogen isotope fractionation as salinity increased to light availability, which was likely lower in the particle rich mixing zone at the end of the estuary, and potentially as well to multiple sources of fatty acids in the CRE.     

Hydrologic implications of the isotopic kinetic fractionation of open-water evaporation

The kinetic fractionation of open-water evaporation against the stable water isotope H_2 ~(18)O is an important mechanism underlying many hydrologic studies that use ~(18)O as an isotopic tracer. A recent in-situ measurement of the isotopic water vapor flux over a lake indicates that the kinetic effect is much weaker(kinetic factor 6.2‰) than assumed previously(kinetic factor14.2‰) by lake isotopic budget studies. This study investigates the implications of the weak kinetic effect for studies of deuterium excess-humidity relationships, regional moisture recycling, and global evapotranspiration partitioning. The results indicate that the low kinetic factor is consistent with the deuterium excess-humidity relationships observed over open oceans.The moisture recycling rate in the Great Lakes region derived from the isotopic tracer method with the low kinetic factor is a much better agreement with those from atmospheric modeling studies than if the default kinetic factor of 14.2‰ is used. The ratio of transpiration to evapotranspiration at global scale decreases from 84±9%(with the default kinetic factor) to 76±19%(with the low kinetic factor), the latter of which is in slightly better agreement with other non-isotopic partitioning results.     

Light carbon isotope events of foraminifera attributed to methane release from gas hydrates on the continental slope,northeastern South China Sea

In 2013, the China Geological Survey and Guangzhou Marine Geological Survey conducted the second Chinese gas hydrate expedition in the northern South China Sea(SCS) and successfully obtained visible gas hydrate samples. Five of the thirteen drilling sites were cored for further research. In this work, Site GMGS2-08 is selected for the stable isotopic analysis of foraminifera present in the boreholes in order to reveal the carbon isotopic characteristics of the foraminifera and their response to methane release in the gas hydrate geological system. Our results show that the methane content at Site GMGS2-08 is extremely high, with headspace methane concentrations up to 39300 μmol L~(-1). The hydrocarbon δ~(13)C values, ranging from-69.4‰ to-72.3‰ PDB, distinctly indicate biogenic generation. Based on the δD analytical results(~(-1)83‰ to~(-1)85‰ SMOW), headspace methane is further discriminated to be microbial gas, derived from CO_2 reduction. By isotopic measurement, five light δ~(13)C events are found in the boreholes from Site GMGS2-08, with foraminiferal δ~(13)C values being apparently lower than the normal variation range found in the glacial-interglacial cycles of the SCS. The δ~(13)C values of benthic Uvigerina peregrina are extremely depleted(as low as~(-1)5.85‰ PDB), while those of planktonic Globigerinoides ruber reach-5.68‰ PDB. Scanning electron micrograph(SEM) studies show that foraminiferal tests have experienced post-depositional alteration, infilled with authigenic carbonate, and the diagenetic mineralization is unlikely to be related to the burial depths. The correlation calculation suggests that the anaerobic oxidation of organic matter has only weak influences on the δ~(13)C composition of benthic foraminifera. This means that the anomalous δ~(13)C depletions are predominantly attributed to the overprinting of secondary carbonates derived from the anaerobic oxidation of methane(AOM). Furthermore, the negative δ~(13)C anomalies, coupled with the positive δ18O anomalies observed at Site GMGS2-08, are most likely the critical pieces of evidence for gas hydrate dissociation in the geological history of the study area.     

Carbon isotope fractionation during CH4 transport in paddy fields

In this study,to further promote the application of the stable carbon isotope natural abundance(SCINA)method to the study of CH4in paddy fields in China,field experiments were carried out to investigate carbon isotope fractionation during CH4transport in both rice-and non-rice-growing seasons.More importantly,two new methods for the measurement of the CH4transport fractionation factor(εtransport)in paddy fields were introduced.The results indicated that the closed chamber+syringe method was much better for the determination ofεtransport during the non-rice-growing season.Presently,εtransport was calculated using theδ13C value of the CH4emitted from a rice field minus that of the CH4in the floodwater(–6.7‰to–3.0‰).In addition,there were three methods available for estimatingεtransport during the rice-growing season:deduction of theδ13C value of the CH4in the floodwater from that of the CH4emitted from the field(–16.6‰to–15.2‰);deduction of theδ13C value of the CH4in the soil pore water from that of the CH4emitted from the field(–13.2‰to–1.1‰);and deduction of theδ13C value of the CH4in the aerenchyma of plants from that of the CH4emitted from plants(–16.3‰to–10.9‰).Unfortunately,the first two methods showed relatively large uncertainties.Only the last one,the dividing+cutting method,was not only scientific and reliable but also provided accurate measurements.